The present invention relates to a process for production of ultrafine nickel powder in which ultrafine nickel powder having an average particle diameter of 1.0 xcexcm or less can be produced by reducing raw material gas including nickel chloride vapor with hydrogen, and in particular, relates to a technique in which the quality of the ultrafine nickel powder can be improved while the productivity thereof is maintained at a high level.
Conductive metal powders such as nickel, copper, silver, palladium, etc., are useful for internal electrodes in multilayer ceramic capacitors, and in particular, since nickel powder, which is a base metal, is inexpensive, such application has recently attracted attention. As a process for production of such a nickel powder, a process in which nickel chloride vapor is generated and is reduced with hydrogen charged into a reducing furnace is known. In addition, multilayer ceramic capacitors generally have a construction such that ceramic dielectric layers and metallic layers used for internal electrodes are alternately laminated. Recently, reduced thickness and reduced resistance in the internal electrode, etc., are required for miniaturization and capacity increase of the capacitors, and therefore, the average particle diameter of the ultrafine powders is preferably 1.0 xcexcm or less, more preferably 0.5 xcexcm or less, and most preferably 0.1 to 0.4xcexcm.
In order to reduce the particle diameter of the nickel powder, it is necessary that the residence time of the nickel chloride vapor in hydrogen be shortened, and in addition, it is necessary that the nickel powder be formed so as to be as spherical as possible, that the particle diameter thereof be made uniform, and that the desired particle diameter be obtained. Furthermore, in order to increase the production yield of the nickel powder, it is effective for the flow rate of raw material gas fed into the reducing furnace to be increased or for the partial pressure of the nickel chloride vapor in the raw material gas to be increased; however, stabilization of quality and further improvement thereof are then difficult.
Therefore, an object of the present invention is to provide a process for production of ultrafine nickel powder in which the following targets can be met.
{circle around (1)} Ultrafine nickel powder is produced in which the average particle diameter thereof is preferably 1.0 xcexcm or less, and more preferably 0.1 to 0.4 xcexcm.
{circle around (2)} Qualities such as uniformity of shape and particle diameter of the ultrafine nickel powders are improved, while manufacturing efficiency is maintained at a high level.
The inventors have performed intensive research with regard to the conditions under which the raw material gas is fed into the reducing furnace. As a result, they have discovered suitable conditions which can meet the above targets. That is, a first process for production of ultrafine nickel powder, in which ultrafine nickel powders are produced by vapor-reducing nickel chloride vapor, is characterized in that raw material gas having a partial pressure of nickel chloride vapor within a range from 0.2 to 0.7 is fed into a reducing furnace, and the nickel chloride vapor is reduced with hydrogen while flowing the raw material gas in this reducing furnace at a space velocity (SV) within a range from 0.02 to 0.07 secxe2x88x921.
In addition, a second process for production of ultrafine nickel powder, in which ultrafine nickel powders are produced by vapor-reducing nickel chloride vapor, is characterized in that hydrogen is discharged from a first outlet nozzle provided at an inlet nozzle of a reducing furnace, raw material gas having a partial pressure of nickel chloride vapor within a range from 0.2 to 0.7 is simultaneously discharged from a second outlet nozzle provided so as to surround the first outlet nozzle, and the nickel chloride vapor is reduced with hydrogen while flowing the raw material gas in this reducing furnace at a space velocity (SV) within a range from 0.02 to 0.07 secxe2x88x921.
More preferred embodiments of the above first or second production processes are as follows.
{circle around (1)} Raw material gas having a partial pressure of nickel chloride vapor within a range from 0.3 to 0.7 is fed into a reducing furnace and the nickel chloride vapor is reduced with hydrogen while flowing the raw material gas in the reducing furnace at a space velocity (SV) within a range from 0.025 to 0.07 secxe2x88x921 .
{circle around (2)} In order to obtain ultrafine nickel powders having an average particle diameter within a range from 0.1 to 0.2 xcexcm, raw material gas having a partial pressure of nickel chloride vapor within a range from 0.25 to 0.6 is fed into a reducing furnace and the nickel chloride vapor is reduced with hydrogen while flowing the raw material gas in this reducing furnace at a space velocity (SV) within a range from 0.03 to 0.07 secxe2x88x921, and it is preferable that raw material gas having a partial pressure of nickel chloride vapor within a range from 0.3 to 0.55 be fed into a reducing furnace and that the nickel chloride vapor be reduced with hydrogen while flowing the raw material gas in the reducing furnace at a space velocity (SV) within a range from 0.035 to 0.07 secxe2x88x921.
{circle around (3)} In order to obtain ultrafine nickel powders having an average particle diameter within a range from 0.25 to 0.4 xcexcm, raw material gas having a partial pressure of nickel chloride vapor within a range from 0.3 to 0.7 is fed into a reducing furnace and the nickel chloride vapor is reduced with hydrogen while flowing the raw material gas in the reducing furnace at a space velocity (SV) within a range from 0.02 to 0.06 secxe2x88x921, and it is preferable that the raw material gas having a partial pressure of nickel chloride vapor within a range from 0.3 to 0.7 be fed into the reducing furnace and that the nickel chloride vapor be reduced with hydrogen while flowing the raw material gas in the reducing furnace at a space velocity (SV) within a range from 0.03 to 0.06 secxe2x88x921.
{circle around (4)} Raw material gas is discharged from a second outlet nozzle to a reducing furnace at a linear velocity within a range from 0.5 to 5.0 m/second.
{circle around (5)} Hydrogen is discharged from a first outlet nozzle provided at an inlet nozzle of a reducing furnace, and raw material gas is discharged from a second outlet nozzle provided around the first outlet nozzle. At this time, hydrogen at 30 to 100 mol % of the theoretical amount required to reduce nickel chloride vapor is discharged from the first outlet nozzle.
In the following, preferred embodiments of the present invention will be explained in detail. Terms used in the present description are defined as follows.
{circle around (1)} xe2x80x9cRaw material gasxe2x80x9d refers to a gas in which nickel chloride vapor is diluted with inert gas and/or halogen gas such as chlorine gas and which is a mixture as a raw material to be reduced. Inert gas or halogen gas acts to dilute the nickel chloride vapor and/or as a carrier thereof. As the inert gas, nitrogen gas or argon gas is generally employed, and in addition, the gas can also be employed with halogen gas in combination.
{circle around (2)} The xe2x80x9cpartial pressure of nickel chloride vaporxe2x80x9d refers to the mole percentage of the nickel chloride vapor occupied in a mixture of nickel chloride vapor with inert gas and/or halogen gas.
{circle around (3)} xe2x80x9cSpace velocityxe2x80x9d is indicated by SV (space velocity; units: secxe2x88x921) and refers to a ratio of feeding speed (liter/second; conversion at reduction temperature and at 1 atm) of nickel chloride vapor fed into a reducing furnace to volume V (liters) of a reacting portion in the reducing furnace (volume of a space from an inlet nozzle portion of raw material gas to a cooling portion for cooling formed ultrafine nickel powder). Although the nickel chloride vapor is fed as a mixture of inert gas and/or halogen gas, SV is the value for nickel chloride excepting the inert gas.
{circle around (4)} xe2x80x9cLinear velocityxe2x80x9d refers to the discharging speed (m/second; conversion at reduction temperature) of raw material gas in the case in which the raw material gas is fed from a second outlet nozzle to a reducing furnace.
As a process for production of nickel chloride vapor which is a component of raw material gas to be reduced, a process in which solid nickel chloride is evaporated by heating, or a process in which nickel metal is brought into contact with chlorine gas, thereby converting it into a metal chloride, can be employed. In particular, the latter process is preferably adopted in the present invention since the production amount of nickel chloride is easily controlled by feeding a set amount of chlorine. As raw material gas fed into the reducing furnace in the present invention, a mixture of nickel chloride vapor with halogen gas and/or an inert gas is preferred. The partial pressure of nickel chloride vapor is preferably 0.2 to 0.7, is more preferably 0.25 to 0.7, and is most preferably 0.3 to 0.7. The range of such partial pressures is a preferable aspect in the case in which an objective ultrafine nickel powder having qualities such as particle diameter, uniformity thereof, shape, crystallinity, sinterability, etc., is produced.
B-1. Overall Composition
FIG. 1 shows an example of a reducing furnace 10 used in the present invention; however, the present invention is not limited to this. At the top of the reducing furnace 10, a raw material gas feeding nozzle 30 connected with a raw material gas feeding pipe 42 is provided, and in addition, a hydrogen feeding pipe 20 is provided at another portion. Furthermore, a cooling gas feeding pipe 11 is provided. A space between a tip (shown by 13a in the figure) of the raw material gas feeding nozzle 30 and a position (shown by 13b in the figure) of the cooling gas feeding pipe 11 is a reaction portion 12. The ultrafine nickel powder produced by a reductive reaction is conveyed to a separation and collection process and to a purification process with surplus hydrogen and by-product hydrogen chloride.
B-2. Feeding Process for Raw Material Gas and Hydrogen
The raw material gas discharging nozzle 30 may be a single pipe, as is shown in FIG. 1, and may branch into two or more branches. The discharging speed of the raw material gas from a raw material gas outlet nozzle, that is, the linear velocity, is desirably set for 0.5 to 5.0 m/second (calculated value converted at the reduction temperature). In the case in which the line velocity is above this range, the reductive reaction becomes nonuniform.
In order to satisfy both productivity and quality requirements for the ultrafine nickel powder, a double-pipe structure (often referred to as a xe2x80x9cmulti-nozzlexe2x80x9d) which provides a hydrogen discharging nozzle 24 in the raw material gas discharging nozzle 30, as is shown in FIG. 2, is preferred. Thus, the reductive reaction for nickel chloride can thereby be carried out more efficiently. As another aspect, nozzles in which multiple raw material gas outlet nozzles are divided around the hydrogen discharging nozzle 24 at the center may be used. According to such an arrangement, nickel chloride vapor is fed from the raw material gas outlet nozzle extremely stably, uniformly, and efficiently so as to react with hydrogen, and ultrafine nickel powder in which the particle diameter distribution is small can thereby be obtained even at high partial pressures of nickel chloride vapor.
B-3. Feeding Amount of Hydrogen
The total amount of hydrogen fed into the reducing furnace is a theoretical amount (chemical equivalent) or more, which is necessary for reducing nickel chloride in the raw material, and specifically, hydrogen of 110 to 200 mol % of the theoretical amount is fed. In the case in which the double-pipe nozzle is used, as shown in FIG. 2, it is preferable, in order to accomplish the object of the present invention, that hydrogen of 30 to 100 mol % of the theoretical amount be fed from the hydrogen discharging nozzle 24 provided at the center and that the remainder which is required be fed from the hydrogen feeding pipe 20 so that the total amount is 110 to 200 mol %. Although there is no problem even if hydrogen is fed above 200 mol % of the theoretical amount, this case is economically inferior. As a preferable aspect, it is particularly effective that 40 to 90 mol % of the theoretical amount be fed from the hydrogen discharging nozzle 24 using the double-pipe shown in FIG. 2, and that 30 to 90 mol % thereof be separately fed from the hydrogen feeding pipe 20, so that the total hydrogen feeding amount is 110 to 180 mol % of the theoretical value.
B-4. Reaction Condition and Space Velocity
The reductive reaction in the reducing furnace is carried out in the reaction portion 12 at 950 to 1150 xc2x0 C. When raw material gas having a partial pressure of nickel chloride vapor within a range from 0.2 to 0.7 is fed from the raw material gas outlet nozzle to the reducing furnace, nickel chloride vapor immediately brings into contact with hydrogen, and a core of nickel is formed and grows. Then, it is rapidly cooled by feeding inert gas from the cooling gas feeding pipe 11 provided at the lower portion of the reducing furnace, etc., and growth thereof is stopped. The ultrafine nickel powder produced by such a procedure is conveyed to a separation and collection process.
In the present invention, it is important to combine the partial pressure of nickel chloride vapor in the raw material gas with a setting of 0.02 to 0.07 secxe2x88x921 for the space velocity (SV) of the nickel chloride vapor in the reaction portion 12 from the outlet nozzle of the raw material gas feeding nozzle 30 to the cooling portion. In the case in which the space velocity (SV) is below 0.02 secxe2x88x921, manufacturing efficiency is extremely low. In contrast, in the case in which it is above 0.07 secxe2x88x921, the quality of the ultrafine nickel powder is tends to be unstable. The space velocity (SV) is preferably 0.025 to 0.07 secxe2x88x921, if conditions are further limited from this viewpoint.
FIG. 3 shows the relationship between partial pressure of nickel chloride vapor and space velocity (SV) thereof to the average particle diameter of the produced ultrafine nickel powder. As is apparent from FIG. 3, in order to control the average particle diameter, ranges of partial pressure of nickel chloride vapor in raw material gas and space velocity (SV) are set as mentioned above, and ultrafine nickel powder having an average particle diameter within a range from 0.1 to 0.2 xcexcm or an average particle diameter within a range from 0.25 to 0.4 xcexcm can thereby be selectively produced.
{circle around (1)} In particular, in order to produce ultrafine nickel powder having an average particle diameter within a range from 0.1 to 0.2 xcexcm, raw material gas having a partial pressure of nickel chloride vapor within a range from 0.25 to 0.6 is fed into a reducing furnace and the nickel chloride vapor is reduced with hydrogen while flowing the raw material gas in the reducing furnace at a space velocity (SV) within a range from 0.03 to 0.07 secxe2x88x921. It is more preferable that raw material gas having a partial pressure of nickel chloride vapor within a range from 0.3 to 0.55 be fed into a reducing furnace and that the nickel chloride vapor be reduced with hydrogen while flowing the raw material gas in this reducing furnace at a space velocity (SV) within a range from 0.035 to 0.07 sec1.
{circle around (2)} In order to produce ultrafine nickel powder having an average particle diameter within a range from 0.25 to 0.4 xcexcm, raw material gas having a partial pressure of nickel chloride vapor within a range from 0.3 to 0.7 is fed into a reducing furnace and the nickel chloride vapor is reduced with hydrogen while flowing the raw material gas in the reducing furnace at a space velocity (SV) within a range from 0.02 to 0.06 secxe2x88x921. It is more preferable that raw material gas having a partial pressure of nickel chloride vapor within a range from 0.3 to 0.7 be fed into a reducing furnace and that the nickel chloride vapor be reduced with hydrogen while flowing the raw material gas in this reducing furnace at a space velocity (SV) within a range from 0.03 to 0.06 secxe2x88x921.
{circle around (3)} Even if the average particle diameter is the same, in the case in which the partial pressure of nickel chloride vapor is low, or in the case in which the space velocity (SV) is small, crystallinity of the produced ultrafine nickel powder is superior and the below-described sinterability is also improved. In this case, since productivity is lowered, partial pressure and space velocity (SV) are appropriately set in consideration of a balance of quality and properties.
As a more preferable aspect, hydrogen is brought into contact with raw material gas and is simultaneously discharged in the reducing furnace, and a reductive reaction is carried out at the above partial pressure of nickel chloride vapor in raw material gas and a space velocity (SV) thereof.